The early events after infection with T. gondii result in innate resistance that prevents infection overwhelming the host before development of protective T cell responses. Interleukin-12 (IL-12) is central to both the innate, natural killer (NK) cell mediated, mechanism of resistance and the development of protective T cell responses. These effects of IL-12 are due to its ability to stimulate NK and T cells to produce IFN-gamma, the major mediator of resistance to T. gondii. However, the ability of IL-12 to stimulate IFN-gamma is dependent on other cofactors which can enhance or inhibit IL-12 mediated events. Understanding how the cellular immune system is regulated during the initiation of an immune response to infection could lead to the development of new approaches to boost the cellular immune response which would be useful in treatment of opportunistic infections, cancer and vaccine design. This proposal has 3 specific aims which focus on the factors that regulate the protective effects of IL-12 during the early immune response to T. gondii. 1. How does the B7-CD28/CTLA-4 interaction regulate NK cell responses during infection? Infection of mice with T. gondii leads to the expression of CD28 by NK cells. The interaction of CD28 with B7 molecules (found on macrophages) enhances IL-12-driven NK cell production of IFN-gamma. We aim to identify the stimulus for NK cells to express CD28 during infection and to differentiate the role of B7-1 and B7-2 in the regulation of NK responses during infection. In addition, our demonstration that activated NK cells express CTLA-4 suggest a role for this ligand for B7 in regulation of NK cell activity. Since CTLA-4 is an important inhibitor of CD28-induced T cell responses, we will determine if stimulation through CTLA-4 can inhibit NK cell function in vitro and in mice infected with T. gondii. 2. What is the role of IGIF in the early immune response to T. gondii? IGIF is a newly described cytokine which, when used in combination with IL-12, results in remarkable levels of NK and T cell production of IFN-gamma. Infection of SCID mice with T. gondii results in increased expression of mRNA for IGIF and pre- treatment with IGIF reduces the parasite load in these mice. These data suggest an important role for IGIF in the regulation of NK cell responses to T. gondii. We will determine the basis for the protective effects of IGIF treatment and the role of endogenous IGIF in NK and T cell mediated resistance to T. gondii. 3. How does IL-10 regulate the early immune response during toxoplasmosis? The production of IL-10 (an inhibitor of cell-mediated immunity) in mice infected with T. gondii has been associated with increased susceptibility to this infection. However, challenge of IL-10 deficient mice with T. gondii results in early mortality due to the development of a "shock" reaction, characterized by high levels of IL-12 and the production of remarkable levels of IFN-gamma by CD4+T cells. We will analyze the mechanisms that underlie this pathogenic reaction and identify the T cell subsets involved in this "shock" reaction.